Two-dimensional radiation detector

ABSTRACT

A two-dimensional, pixellated, monolithic semiconductor radiation detector, in which each detector pixel is essentially a perpendicular mode detector. This is achieved by an arrangement of anode spots, one for each pixel located on the flux-exposed front surface of the detector substrate, surrounding by a cathode array preferably in the form of a network of lines, such that the field between the anodes and cathodes on this front surface has a major component in the direction parallel to the surface, and hence perpendicular to the incident photon flux. The conductivity of the substrate is high near this front surface, since this is where the highest level of absorption of photons takes place, and a significant photoconductive current is thus generated between cathodes and anodes. The conductivity is proportional to the incoming photon flux, and decays exponentially with depth into the detector. Since all of the conduction paths are in parallel to each other, the resultant conductance between each anode and its surrounding cathode is the summation of all those conductances.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor photondetector arrays, having the bias field perpendicular to the direction ofincident radiation, especially for use in imaging applications.

BACKGROUND OF THE INVENTION

Current-mode detectors, operating in the photoconductive mode, aregenerally used whenever the flux of impinging photons is too intense toallow the counting of single photons. This is generally the situation inX-ray imaging. In such situations, the practical method for measuringthe intensity of the radiation is to measure the average currentstemming from charges created by absorption of a large number ofphotons. For example, the detectors presently used by ComputerizedTomography (CT) imaging systems are scintillators coupled tophotodiodes, designated hereinafter SPD's. Each X-ray photon absorbedwithin the scintillator creates a pulse of light, which is then detectedby the photodiode and converted to multiple electron-holes pairs, withthe number of those pairs being proportional to the energy of theabsorbed X-ray photon. These charges are then sampled over a timeframewhich is much longer than the average time between individual events ofX-ray photon absorption within the scintillator. The read-out signal isthus proportional to the average current from the photodiode, which isproportional to the X-ray flux hitting the scintillator.

Two decades ago, there was already a trend in CT imaging technology toreplace the SPD by all-semiconductor detectors. One of the first suchexamples is described in the article by P. A. Glasgow et al, entitled“Aspects of Semiconductor Current Mode Detectors for X-ray ComputerTomography”, published in IEEE Transactions on Nuclear Science, Vol.NS-28, pp. 563-571, February 1981. Such detectors have the inherentadvantage of operating in a direct conversion mode, i.e., the X-rayphoton absorbed within the semiconductor volume is directly converted toelectron-hole pairs. Such a direct conversion is an order-of-magnitudemore effective than the previously used indirect conversion process ofX-ray photons to light within the scintillator, and light toelectron-hole pairs within the photodiode. For instance, in a detectormade of CdTe or CdZnTe (CZT), the number of electron-hole pairs createdby an absorbed X-ray photon of energy E₀ is approximately E₀/4.4 eV,whereas in a conventional SPD detector, the number is in the rangetypically of from E₀/30 eV to E₀/60 eV only. Semiconductor detectors arenot only more efficient, but they also allow the fabrication of arraysof detectors over a monolithic slab of the semiconductor, with pixels ofdesired dimensions, especially of very small dimensions which arepractically impossible to fabricate in conventional SPD structures. Thisis a very important advantage of semiconductor detectors over SPDdetectors, since the trend in CT imaging technology is presently formany more channels of detection, using much smaller detectors to allowbetter spatial resolution. This trend is only feasible currently byreplacing the SPD detectors with semiconductor detectors such as CdTe orCZT.

Such semiconductor detector operate in the current-mode by utilizing thephotoconductive effect. When the X-ray flux is absorbed within thephotoconductor (PC), the conductivity of the semiconductor essentiallychanges from its dark-value, determined by the thermal excitation ofelectrons within the bulk of the PC, to a higher conductivity,determined by the higher density of electrons created by the absorbedX-ray photons. Since the PC is kept under a bias voltage between twoelectrodes contacting its bulk, this change in conductivity istranslated into a change in the resultant current, from that of thedark-current to that of the photocurrent.

It has been shown in an article entitled “Possible use of cadmiumtelluride for detection of pulsed X-rays in medical tomography” by E. N.Arkad'eva et al., published in Soviet Physics—Technical Physics, Vol.26, pp. 1122-1125, September 1981, that for the PC detector to behave inan optimal way and to exhibit correct temporal behavior, the X-radiationshould impinge on the PC detector in a direction perpendicular to theelectric field established within the PC. If the X-radiation hits the PCdetector parallel to the electric field, i.e. through the cathode or theanode, there will be temporal overshoot of the PC detector current inresponse to the incidence of an X-ray pulse. Furthermore, the PCdetector photocurrent in this parallel mode is considerably smaller thanthat in the perpendicular mode.

However, according to the current state of detector technology,perpendicular-mode detectors can be fabricated only as rods, with theelectrodes on the narrow sides thereof, and pixellated only in onedimension, namely the length of the rod. Consequently, one of theinherent advantages of using a semiconductor detector, namely, thepossibility of fabricating two-dimensional pixellated monolithic arrays,cannot be realized. Detectors have been described in whichone-dimensional arrays, i.e. pixellated rods, are stacked side by sideto form a two dimensional array, but such an array is difficult tofabricate, and is thus costly. There therefore exists an important needfor a two-dimensional pixellated monolithic array, capable of being usedin the perpendicular mode, with the radiation impinging on the array ina direction perpendicular to the direction of the application of thebias field.

The disclosures of each of the publications mentioned in this sectionand in other sections of the specification, are hereby incorporated byreference, each in its entirety.

SUMMARY OF THE INVENTION

The present invention seeks to provide a novel, two-dimensional,pixellated, monolithic semiconductor radiation detector, in which eachdetector pixel is essentially a perpendicular mode detector. This ispreferably achieved by an arrangement of anode spots surrounding by acathode array preferably in the form of a network of lines, deposited onthe flux-exposed front surface of the detector substrate, such that thefield between the anodes and cathodes has a major component in thedirection parallel to the surface, and hence perpendicular to theincident photon flux. The conductivity of the substrate is high nearthis front surface, since this is where the highest level of absorptionof photons takes place, and therefore, a good level photoconductivecurrent is generated between the front surface cathodes and anodes. Thiscurrent is preferably measured in each pixel by means of currentamplifiers connected to each anode. The conductivity is proportional tothe incoming photon flux, and decays exponentially with depth into thedetector. Since all of the conduction paths are in parallel to eachother, the resultant conductance between each anode and its surroundingcathode structure is the summation of all those conductances. Thecurrent measured by the current amplifier is thus proportional to thetotal X-ray flux incident within that pixel.

According to an additional preferred embodiment of this invention, asecond set of anodes and cathodes are located on the rear surface,thereby increasing the efficiency of the detector by collectingelectrons generated at the rear of the detector, which would otherwisenot be measured by the front set of electrodes.

Furthermore, by connecting all of the front surface electrodes togetherelectrically, to form a single cathode structure, the rear anodes can beused for single event photon counting at low incident flux levels, withall the advantages which this measurement offers at low flux levels. Thetransition from photoconductive measurements to single photon countingmeasurements can be performed automatically according to the flux leveldetected by the measurement system.

The perpendicular mode field configuration can be advantageously used,according to further preferred embodiments of the present invention, toenable the execution of photon counting measurements at flux levelssubstantially higher than those achievable using the prior art parallelfield configuration. The efficiency of prior art photon countingdetectors falls drastically with increasing incident flux, because thehigh conductivity regions generated near the front surface of suchdetectors essentially prevents the production of an electric field inthose regions, and therefore prevents the electrons generated thereinfrom being attracted across the conductivity gradient towards theircollection anodes. With the field applied perpendicularly to the fluxdirection, and hence perpendicular also to the conductivity gradient,the field seen by electrons generated in the substrate is independent ofthe depth of penetration, and such electrons are therefore allinfluenced by the same approximate level of perpendicular field,independently of the conductivity of the region in which they aregenerated. According to these additional embodiments of the presentinvention, at flux levels which, according to prior art detectortechnology, mandated the use of photoconductivity measurements, ittherefore becomes possible to use the methods of photon counting, withtheir concomitant advantages, rather than photoconductivitymeasurements.

There is thus provided in accordance with a preferred embodiment of thepresent invention, a two dimensional planar radiation detectorcomprising a semiconductor substrate having a first surface for exposureto the radiation, and an array of anodes and cathodes disposed on thesurface, such that the electric field produced between at least one ofthe anodes and its associated cathodes has a substantial componentperpendicular to the direction of impingement of the radiation.

The detector may preferably be adapted to be aligned relative to theradiation such that the substantial component of the electric fieldproduced between at least one of the anodes and its cathode isessentially perpendicular to the direction of impingement of theradiation. In the above mentioned detectors, the array of cathodes maybe in the form of a net of cathodes, having an anode within each cell ofthe net. At least one of the cathodes may preferably be essentiallysquare in shape, and at least one of the cathodes may be unclosed inform.

There is further provided in accordance with yet another preferredembodiment of the present invention, a radiation detector as describedabove, wherein the radiation is operative to generate a conductiveregion in the substrate, close to the first surface, such that aphotocurrent flows between at least one of the anodes and its cathode.The photocurrent is preferably dependent on the intensity of theradiation.

In accordance with still another preferred embodiment of the presentinvention, in the radiation detector as described above, the electricfield may be such that the substantial component is essentially constantin the direction perpendicular to the first surface. The radiation isthen preferably operative to generate a conductive region in thesubstrate, close to the first surface, and an electron generated in theconductive region sees the same substantial component of the field, asan electron generated outside of the conductive region.

Any of the radiation detectors described above, according to morepreferred embodiments of the present invention, may also comprise asecond surface distant from the first surface, on which second surfacemay be disposed a second array of anodes and cathodes such that asubstantial component of the electric field produced between at leastone of the anodes and its cathode of the second array is essentiallyparallel to the second surface. The second array of anodes and cathodesis preferably operative to measure a photocurrent arising from electronsgenerated in the substrate and not measured by the array of anodes andcathodes on the first surface. The ratio between the photocurrents onthe first and the second surfaces may then preferably be used to providean indication of the hardness of the radiation.

There is further provided in accordance with still another preferredembodiment of the present invention, a radiation detector as describedabove, and also comprising a second surface distant from the firstsurface, and wherein on the second surface is disposed an electrodemaintained at a negative potential relative to the front surface anodesand cathodes, and wherein the electrode is operative to repel electronsgenerated near the rear surface back towards the front surface anodesand cathodes.

In accordance with further preferred embodiments of the presentinvention, there are also provided radiation detectors as describedabove, and also comprising a second surface distant from the firstsurface, and wherein on the second surface is disposed an array ofanodes, operative to perform photon counting measurements on electronsnot measured by the front surface array of anodes and cathodes.

The substrate in any of the above mentioned radiation detectors ispreferably cadmium zinc telluride.

There is also provided in accordance with yet a further preferredembodiment of the present invention, a method of performing photoncounting measurements on a radiation flux, comprising the steps ofproviding a semiconductor substrate having a first surface for exposureto the radiation flux, disposing on the first surface an array of anodesand cathodes, such that a substantial component of the electric fieldproduced between at least one of the anodes and its cathode isessentially parallel to the first surface, and measuring at the at leastone of the anodes, the charge arising from individual photon absorptionsin the substrate. The photon counting may be performed at flux levelswhich generate significant conductive regions in the substrate.

Furthermore, according to another preferred embodiment of this method, asecond plurality of anodes and cathodes may be disposed on the secondsurface of the substrate, such that an electric field produced betweenat least one of the anodes and an associated cathode of this secondplurality has a component parallel to the second surface, and the chargearising from an individual photon absorption in the substrate ismeasured by at least one of the second plurality of anodes.

Additionally and preferably, at least one second anode electrodemaintained at a positive potential relative to the plurality of firstsurface anodes and cathodes may be disposed on the second surface, andthe the charge arising from an individual photon absorption in thesubstrate is measured by at least one of the at least one second anodeelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1A is a schematic representation of the cross section of a priorart, parallel-mode, photoconductive detector, with the photon fluximpinging on the detector in the same direction as the bias field;

FIG. 1B is a schematic representation of the cross section of a priorart, perpendicular-mode, photoconductive detector, with the photon fluximpinging on the detector in a direction perpendicular to the biasfield;

FIG. 2 is a schematic drawing of a prior art stack of perpendicular-moderod detectors, providing a two-dimensional perpendicular-mode detectorarray;

FIG. 3 is a schematic cross-sectional drawing of a planar monolithictwo-dimensional perpendicular-mode detector array, constructed andoperative according to preferred embodiment of the present invention;

FIG. 4 is a plan view of the detector array of FIG. 3, showing apreferred arrangement of the electrode structure;

FIG. 5 is a schematic cross-sectional illustration of a furtherpreferred embodiment of the present invention, similar to that shown inFIGS. 3 and 4, but with a second set of cathodes and anodes on the backface of the detector crystal;

FIG. 6 is a schematic cross-sectional illustration of a furtherpreferred embodiment of the present invention similar to that shown inFIGS. 3 and 4, but with a continuous cathode with a negative appliedvoltage disposed on the rear surface to reject any electrons generatednear or reaching the vicinity of the rear surface; and

FIG. 7 is a schematic cross-sectional illustration of a furtherpreferred embodiment of the present invention similar to that shown inFIGS. 3 and 4, but with an additional array of anode spots disposed onthe rear surface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1A, which schematically illustrates thecross-section of a prior art, parallel-mode, photoconductive detector10, with the photon flux 12 impinging on the detector in the samedirection as the bias field. The bias field is applied between anode 14and cathode 16 on opposite faces of the detector crystal. The effect ofthe incident flux of photons is to produce a low resistance layer 18 inthe region where the absorption takes place, but in the remainder of thedetector depth 20, the resistivity remains at its original high level.As a result, the current through the detector remains limited by thehigh resistance region, and the resulting sensitivity to the photonillumination is low. This explains a major disadvantage of the parallelmode of operation of such photoconductive detector arrays.

Reference is now made to FIG. 1B, which schematically illustrates thecross-section of the end view of a prior art, perpendicular-mode,photoconductive detector 22, with the photon flux 12 impinging on thedetector in a direction perpendicular to the bias field. The photon flux12 produces the same low resistivity layer 18 as in the embodiment shownin FIG. 1A, with the remainder of the detector crystal 20 retaining itsdark-level high resistivity. The bias field is applied between anode 24and cathode 26 on opposite faces of the detector crystal, but unlike theembodiment shown in FIG. 1A, the direction of the bias field isperpendicular to the incident flux direction. As a result, even thoughin the example shown in FIG. 1B, the bulk of the detector crystal 20 hasa high resistivity, there is a reasonable photoconductive currentbetween anode and cathode through the low resistivity layer 18. Thesensitivity of this detector arrangement is therefore generallysignificantly higher than that of the parallel mode detector scheme ofFIG. 1A. However, the geometry of the detector shown in FIG. 1B is onedimensional, in a direction perpendicular to the end face shown in theplane of the drawing, and it would be very advantageous to provide aperpendicular-mode detector being two dimensional in both directionsacross the plane of the incident photon flux 12.

Reference is now made to FIG. 2, which is a schematic drawing of a priorart stack of perpendicular-mode rod detectors such as those shown inFIG. 1B. Each of the rod detectors 30 has a one-dimensional x-axis arrayof anodes 34 along one surface, and a cathode electrode 36 on theopposite surface. The rod detectors are aligned such that the photonflux 32 is incident on each rod detector in the z-direction, which isperpendicular to the direction of the electric field between the anodeand cathode surfaces. In this way, each rod detector has the highsensitivity characteristic of perpendicular-mode detector operation. Thesecond dimension in the plane of the photon flux is provided byproviding an array of such perpendicular-mode rod detectors, stacked inthe y-direction, such that a complete two-dimensional x-y planeperpendicular-mode detector array is provided. The embodiment shown inFIG. 2, though generally better in performance than prior artparallel-mode arrays, is however, complicated to construct, since itinvolves mechanical and electrical assembly by integration of a numberof separate rod detector elements. This makes such an array costly andtime-consuming to manufacture. It would therefore be very advantageousif such a perpendicular mode detector array could be provided using thewidely used and cost-effective planar monolithic fabrication methods.

Reference is now made to FIGS. 3 and 4, which schematically illustrate atwo-dimensional, perpendicular-mode, planar detector array, constructedand operative according to a preferred embodiment of the presentinvention. FIG. 3 is a schematic cross-sectional drawing of the detectorarray, while FIG. 4 is a plan view of the detector array of FIG. 3. Thesame reference numbers are used for the same features in both of thesedrawings. The detector array is preferably constructed by conventionalmonolithic fabrication technology. The material of the detector ispreferably any semiconductor material which is sensitive to the incomingradiation to be imaged, and especially the commonly used cadmiumtelluride or cadmium zinc telluride.

The semiconductor detector array 40 is arranged such that the photonflux 12 to be imaged is incident on one planar face 42. On this face 42,there is deposited a net shaped electrode 44, preferably of squareshape, each square constituting a single pixel of the detector array.The pitch of the net is preferably of the order of 1 mm. or even less,to provide good image resolution over the detector surface. A voltage V2is applied to this net shaped electrode. Inside each square of the netshaped electrode 44, there is deposited a contact dot 46, to each ofwhich is applied a voltage V1. V1 is held at a higher potential than V2,such that the net shaped electrode is operative as the cathode, and eachcentral dot is operative as the anode of its pixel. The currentsdetected in each pixel are measured, preferably by means of currentamplifiers, shown in FIG. 3 as A1, A2, A3 . . . , such that a completetwo-dimensional electronic image of the incident radiation may thus beobtained.

This geometrical arrangement of anodes and cathodes disposed on onesurface of the detector substrate results in a curved field pattern 48between each anode dot and its surrounding cathode. X-radiationimpinging on the planar face 42 of the detector array results in aconductivity gradient in the substrate material, which may thus distortthese conductive paths in comparison with an unradiated high resistivitysubstrate, but the basic curved nature of the conduction path shape ismaintained. Though the trajectories of these conductive paths arecurved, the field produced in the detector has a major componentparallel to the surface 42 of the detector, and hence generallyperpendicular to the incident photon flux, and the detector thusoperates essentially in the perpendicular mode. The paths nearer to theface 42 are more conductive than those deeper in the material, since theconductivity is a function of the absorbed radiation, and the X-rayradiation is absorbed in an exponentially decreasing manner. However,since all of these paths conduct in parallel to each other, theresultant conductance between each anode and its surrounding cathodestructure is the summation of all those conductances. The currentmeasured by the current amplifier, if such is used for the measurement,is thus proportional to the total X-ray flux incident within that pixel.It is thus evident that the arrangement of the electrodes of theembodiment shown in FIGS. 3 and 4 is such as to cause the detector arrayto operate in the perpendicular mode, and in a manner whereby thedetected current provides a generally accurate measure of the incidentphoton flux.

However, in contrast to the prior art detector array shown in FIG. 2,the preferred embodiment of FIGS. 3 and 4, and all the subsequentembodiments shown hereinbelow, all according to the present invention,are capable of being constructed in planar geometry, thus providing theadvantages of perpendicular-mode detection with the cost-effectivenessof monolithic planar fabrication techniques. The embodiment of FIG. 4shows all of the cathode net connected together, such that the whole netcould be described as being a single cathode. Throughout thisspecification, however, and as claimed, the term cathode is understoodto mean that part of the net which is in close electrical fieldproximity to a corresponding anode. The net is thus understood to bemade up of a plurality of cathodes, each providing the opposing negativepotential for a corresponding anode. This combination of an anode withits surrounding cathode then constitutes a detector pixel. Though theembodiment of FIG. 4 shows a net of square cathodes surrounding theanodes, it is to be understood that the invention is not limited to sucha cathode geometry, and is operable with a net of cathodes of any shape,so long as they provide a net transverse field component with respect tothe anodes. Additionally, although the anode is shown as a dot, it is tobe understood that the invention is not limited to such an anode shape,but is operable with any shaped anode, so long as it is within theconfines of its surrounding cathode field. Furthermore, though thecathodes are shown in the embodiment of FIG. 4 as a closed andcontinuous net structure, it is to be understood that they can also haveopenings so long as electrical contact is ensured to every cathode cell,and that a net transverse field is provided between each cathode celland its anode. In addition, although the embodiment of FIGS. 3 and 4 hasbeen shown with a net of cathodes, and an anode inside each cathodecell, it is to be understood that the invention will also operate withV1<V2, such that there is a net of anodes, and a cathode inside eachanode cell.

Reference is now made to FIG. 5, which is a schematic cross sectionillustration of a further preferred embodiment of the present invention.The detector array of FIG. 5 is similar to that shown in FIGS. 3 and 4,except that a similar arrangement of cathodes 50 and anodes 52 are alsoprovided on the back face 54 of the detector crystal 56, remote from thephoton flux 12. The photoconductive effect on the back face 54 is muchsmaller than that on the front face 58, since the X-ray radiation, orother measured radiation, reaching the back face is attenuated by thethickness of the semiconductor wafer. However, according to theembodiment of FIG. 5, this backside signal is not wasted but is added tothe front side signal to increase the total signal measured. Though notshown in FIG. 5, it is understood that the currents collected by theanode dots may preferably be measured by means of current amplifiers.

The back surface signal obtained using the preferred embodiment of FIG.5 can also be used, according to the method of a further preferredembodiment of the present invention, to derive information as to thespectral make-up of the X-ray radiation, or other radiation detected.Since harder (higher energy) radiation penetrates the depth of thedetector crystal further than softer (lower energy) radiation, harderradiation also exhibits a relatively larger back surface signal thanthat of softer radiation. A comparison of the back surface signal withthat of the front surface signal enables information to be obtained asto the spectral make-up of the X-ray radiation. This information isvaluable to CT image interpretation, since X-rays traveling throughthicker body tissues are hardened relative to the original incidentX-ray radiation. This discrimination between front and back surfacesignals can also be facilitated by selection of the comparative anodevoltages applied to the front and back surface anodes, as will bediscussed in detail below in connection with the embodiment of FIG. 6.

Reference is now made to FIG. 6, which is a schematic cross-sectionalillustration of a further preferred embodiment of the present invention.The detector array of FIG. 6 is similar to that shown in FIGS. 3 and 4,except that an additional cathode 60 with an applied voltage V3 isdisposed on the rear surface, remote from the photon flux 12. Thiscathode is preferably a continuous electrode, and the value of V3 ismade sufficiently negative that the cathode 60 acts as a rejectingelectrode to any electrons in its vicinity. For a pixel of dimensions 1mm×1 mm in a 3 mm thick CdZnTe detector, typical values of thesevoltages which have been found to operate satisfactorily are V1=0,V2=−20V, V3=−200V. Thus, an electron 62 produced by impact of anenergetic photon 64 far from the front surface anode-cathodearrangement, which would otherwise not be collected by the front-surfaceanodes, is repelled by the back surface cathode 60, such that it travelsin the direction towards the front surface, where it is collected andmeasured by a front surface anode dot. The use of the rear cathodeembodiment is thus another preferred method for increasing thesensitivity of the single-sided detector shown in the embodiment ofFIGS. 3 and 4.

Though not shown in either FIG. 5 or FIG. 6, it is to be understood thatthe currents collected by the anode dots are preferably measured bymeans of current amplifiers, though, as will be discussed hereinbelow,other measurement schemes may also be preferably used.

The depth in the detector at which an electron is produced from anabsorption event is dependent on the hardness of the incident radiation.The location from which an electron is influenced to move towards thefront surface is dependent on the magnitude of the rear cathode voltageV3 relative to the anode voltage V1 and the front cathode voltage V2.Consequently, according to another preferred method of the presentinvention, V3 can be chosen in order to select the radiation hardness atwhich an electron arising from absorption of that radiation is deflectedforward and detected.

Reference is now made to FIG. 7, which is a schematic cross-sectionalillustration of a further preferred embodiment of the present invention.The detector array of FIG. 7 is similar to the embodiment shown in FIGS.3 and 4, except that an additional array of anode spots 70 is disposedon the rear surface 72, i.e. that remote from the photon flux 12, andpreferably opposite the cathode net lines 74 on the front surface. Alarge positive voltage, typically of the order of 200 volts, is appliedto these rear surface anode spots 70, such that they are attractive toany electrons in their vicinity. According to this preferred embodiment,the array of anodes and cathodes on the front surface operates tomeasure the photon flux by means of the photoconductive mode describedin the embodiment of FIGS. 3 and 4. Though not shown in FIG. 7, it isunderstood that the currents collected by the anode dots on the frontsurface are generally measured by means of current amplifiers. Anyelectrons which get past the front surface field and are not collectedby the front surface anodes, or which are generated by absorption eventsnear the rear surface, are attracted by the large positive voltage onthe rear surface anode dots 70, and are collected there. Since thenumber of such electrons is generally low, they can be measured by meansof the type of circuitry used for individual photon counting. Since theelectrons which do get past the front surface field are generally thosewhich are generated by events in the areas behind the regions of thecathode net lines 74, and therefore out of the field of influence of thefront surface anodes, the rear surface anode dots 70 are preferablylocated directly behind the front surface cathode net lines 74.

The detector in the embodiment of FIG. 7 is therefore operative tosimultaneously perform photoconductive current measurements on the frontsurface and photon counting measurements on the rear surface. The valueof the rear anode voltage, shown in the preferred embodiment of FIG. 7as 200 volts, can be adjusted according to the hardness of radiationthat is to be detected at the rear surface.

According to yet a further preferred embodiment of the presentinvention, the embodiment shown in FIG. 5, in which measurement isperformed on the rear surface also, can be used in another mode ofoperation. In high energy resolution gamma photon imaging, as is commonin Nuclear Medicine Imaging, the pixel size commonly used is of theorder of at least a few millimeters, since the low rate of gamma photonsevents typical to Nuclear Medicine Imaging applications, would notgenerally allow adequate photon statistics using smaller pixels.Therefore, according to this additional preferred mode of operation, thephoton counting measurement on the back surface is performed byintegrating the electrons collected over a group of dots on thissurface, instead of measuring the electrons collected at each dotseparately. A typical embodiment for such a detector with a 1 mm pitchnet dimension, is to take a group of nine such dots and to join themelectronically to form an effective 3 mm×3 mm anode pixel. The potentialof the net on the rear surface can preferably be either floating, orslightly more negative than the rear surface anode, so as to assist indeflecting the electrons towards the dots. Both the net and the dots onthe front surface are preferably set at the same potential, which isnegative relative to that of the dots on the back surface, and thusserves as the common cathode of the detector. Thus, according to thispreferred embodiment, the same detector which is used for high fluxlevel X-ray measurements, can be also be used for high sensitivityNuclear Medicine Imaging applications.

According to further preferred embodiments of the present invention, anyof the embodiments shown in FIGS. 5 to 7, having electrodes on the frontand back surfaces of the detector substrate, and operating in theperpendicular mode, can be used in another mode of operation. Theseembodiments are able to switch from the above-described photoconductivetypes of measurement at high photon flux levels, to photon countingmeasurement at similar or somewhat lower photon flux levels. Thesedetectors are thus designated hybrid measurement photon detectors. Theyare operable by a method, according to another preferred embodiment ofthe present invention, whereby photon counting is useable over a widerrange of incident photon fluxes than is possible under prior art methodsof photon counting. The flux levels at which these novel methods ofphoton counting can be applied is dependent on the pixel size used andthe speed of the electronic circuitry used for the detection.

Photon counting is an advantageous mode of flux measurement compared tothe photoconductive mode of measurement, since the individual countingof each photon absorbed makes the flux determination highly linear, andthe dynamic range of this measurement is thus practically unlimited. Theresultant practical outcome of photon counting is a broader range ofimage gray levels, enabling the attainment of better image contrast andaccuracy, as compared to that achievable in the photoconductive mode.Furthermore, the circuits used for photon counting methods measure thecharge which each individual photon event generates, and since thischarge is dependent on the individual photon energy, photon countingmethods are also generally able to determine the incident photon energyspectrum. However, photon counting methods as applied to prior artdetectors, are only practical at low photon flux levels, as will beexplained hereinbelow.

According to further embodiments of the present invention, theadvantages of photon counting can be utilized at levels of input photonfluxes, in the same detector as is used for perpendicular modephotoconductivity measurements when the input flux is high, by using thefront and rear surface electrode arrays in a different manner for thetwo cases. When the photon flux is high, the photoconductivitymeasurement is performed by the front face anode and cathode electrodes,with or without the assistance of the rear face electrodes, as describedin detail in the previously mentioned embodiments. However, as soon asthe incident flux is at a level low enough to allow efficient photoncounting, circuitry in the detection system is operative to sense thelow flux level, and the detection system is automatically switched tothe photon counting mode. In this mode, the potentials on the cathodenet and the anode dots on the front surface are equalized (V1=V2), and apositive voltage, relative to the front surface potential, is applied tothe anode dots on the rear surface. This positive voltage is designatedV3, where V3>V1, V2. The front face electrodes together serve as acommon cathode, whereas each dot on the rear surface act as thecollecting anode for its specific pixel, and individually counts theabsorbed X-ray photons. The potential of the net on the rear surface canpreferably be set so that the net deflects oncoming electrons towardsthe back face dots, as is well known in the photon-counting art. Thisincreases the efficiency of the counting.

According to the methods of the prior art, at low flux levels photoncounting measurements can be readily performed in the conventionalparallel mode of operation of a detector. At such low flux levels, thereis no significant change in the resistivity of the detector materialfrom its dark value, and the field within the detector is comparativelyundistorted, and reasonably uniform from the front to the back of thematerial. Thus, when a photon, such as an X-ray photon, strikes thedetector on its front cathode face and is absorbed in the bulk, theelectrons generated by the absorption travel quickly under the influenceof this field to the anode on the back surface, from which they arecollected and measured by the photon counting circuitry. In a nuclearmedicine imaging application, where the incoming photon is a high energygamma ray photon, the circuitry also determines the photon energy fromthe number of electrons generated per event.

As the flux level increases, however, as is the case with the X-rayphotons in CT applications, the front of the detector material, wherethe photons are absorbed, becomes increasingly conductive, as describedabove for the photoconductive embodiments of this invention. As aresult, the field profile from front to back of the detector becomesvery non-uniform, with almost zero field at the front end where thematerial is very conductive. As a consequence, electrons generated byphoton absorption in this region remain almost static, and do notreadily reach the back regions of the detector, where the material stillhas its dark-level high resistivity, and where most of the field is thusgenerated, and from where they can be swept by this field to the anodeand measured. Thus, the same mechanism of high conductivity generated inthe regions of high photon flux absorption, that prevents efficientphotoconduction detection in the parallel mode at high photon fluxes,also prevents efficient photon counting measurements at high fluxlevels. This result may be unsurprising, since one of the basicdifferences between photoconductive operation and photon countingoperation lies only in the electronic measurement technique applied inthe two cases. When the electronics is fast enough to discriminatebetween individual photon events, then the measurement is defined as aphoton counting measurement, whereas, when the electronics cannotdiscriminate between electrons arising from different events, becausethe flux is so high, the circuit simply integrates the electron flow,and provides a photoconductivity measurement.

As a result of the foregoing phenomenon, according to more preferredembodiments of the present invention, there are provided methods forperforming photon counting even at high photon fluxes. By operating anyof the perpendicular mode embodiments of FIGS. 3 to 7 in a photoncounting arrangement, the same factors which allowed the photoconductivemeasurements to operate efficiently even at high flux levels, are alsooperative to allow for more efficient photon counting measurements. Thefield non-uniformity is not now in the direction of the depth of thedetector, but in the transverse direction, between the cathode net linesand the anode dots. The curved paths shown in FIG. 3 to representconduction paths for the photoconductive embodiments can, for the photoncounting embodiments, now be considered to represent field lines runningbetween the anode and cathodes. Since the potential difference betweenanode and cathode of the pixels is constant, the electric fieldgenerated along any of the field lines between anode and cathode is ofthe same order of magnitude (since the field lines are of the same orderof length), regardless of the conductivity of the material in which thefield is situated. Therefore, electrons generated near the front of thedetector material, even in a highly conductive region, essentially seeapproximately the same field as that seen by electrons generated deeperin the detector in high resistivity regions, and all are swepttransversely by the anode-cathode field to the anodes, where they arecollected and measured by the photon counting circuitry. Thus, photoncounting can be performed using the above-described perpendicular modeembodiments of the present invention, even at higher photon fluxes,which would prohibit such measurements in prior art parallel modedetectors.

The flux level at which the photon counting mode is possible in all ofthe above described embodiments is determined as that for which thespeed of response of the measuring electronic circuitry allowsindividual measurement of each absorption event for the pixel size used.As an example of this criterion, the highest flux levels currentlydetected in CT measurements is typically of the order of 4×10¹⁰photons/cm²/sec. For a high resolution image with a very small pixelsize, such as 0.15 mm×0.15 mm, the electronic measurement circuit musttherefore be fast enough to measure at a rate of 10 MHz to keep track ofeach individual photon event. The characteristic time-scale ofelectronic pulses resulting from the electrons generated by an X-rayphoton absorption event, which are detected and measured in such CTsystems, is of the order of several nanoseconds, such that theamplification circuitry and components can easily handle the required 10MHz measurement rate.

On the other hand, if a larger pixel size were to be used, such as 1.5mm×1.5 mm, the required speed would be of the order of 1 GHz, a ratewhich is not currently easily achievable in quantitative amplificationand measurement circuitry. Photon counting techniques can, however, beused for the large pixel sizes typically used in nuclear medicine gammaray imaging, since the flux detected is so low that the electronics hasno difficulty in keeping track of the individual events, even in suchlarge pixels.

It can thus be stated that, by use of the methods and devices of thevarious embodiments of the present invention, photon counting can beused, when the conditions enable its use, as a replacement for prior artphotoconduction methods, with all the inherent advantages of photoncounting, on condition that the pixel size is maintained within thelimits which allow the circuitry to keep track of the rate of photonabsorption events occurring in each pixel.

According to these preferred embodiments of the present invention, thereis therefore provided a universal type of photon detector, which isoperable in the same physical configuration at high or low flux levels,for photoconductivity or for photon counting applications, and fornuclear medicine gamma imaging applications, and even including energyresolution measurements. Switching between the different measurementmodalities is performed by selection of the relative potentials appliedto the electrodes, and by selection of the way in which the electrodesare connected, as expounded in detail in connection with theabove-described preferred embodiments of the present invention. All ofthese functions can be performed by means of switching of the electroniccircuitry associated with the measurement of the currents or chargesgenerated by the radiation.

It is appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

1. A radiation detector comprising: a semiconductor substrate having afirst surface for exposure to said radiation; and a plurality of anodesand cathodes disposed on said first surface, such that an electric fieldproduced between at least one of said anodes and an associated cathodehas a component generally parallel to said first surface.
 2. A radiationdetector according to claim 1, wherein said detector is adapted to bealigned relative to said radiation such that said component of theelectric field is generally perpendicular to the direction ofimpingement of said radiation.
 3. A radiation detector according toclaim 1, wherein said cathodes are in the form of a net, having an anodewithin each cell of said net.
 4. A radiation detector according to claim3, wherein at least one of said cathodes is essentially square in shape.5. A radiation detector according to claim 3, wherein at least one ofsaid cathodes is not closed.
 6. A radiation detector according to claim1, wherein said radiation is operative to generate a conductive regionin said substrate close to said first surface, such that a current flowsbetween at least one of said anodes and an associated cathode.
 7. Aradiation detector according to claim 6, wherein said current isdependent on the intensity of said radiation.
 8. A radiation detectoraccording to claim 1, wherein said electric field is such that saidcomponent is essentially constant in the direction perpendicular to saidfirst surface.
 9. A radiation detector according to claim 8, whereinsaid radiation is operative to generate a conductive region in saidsubstrate close to said first surface, and an electron generated in saidconductive region sees said same component of said field as an electrongenerated outside of said conductive region.
 10. A radiation detectoraccording to claim 1 wherein said substrate comprises cadmium zinctelluride.
 11. A radiation detector comprising: a semiconductorsubstrate comprising a first surface for exposure to said radiation anda second surface opposite said first surface; a plurality of anodes andcathodes disposed on said first surface, such that an electric fieldproduced between at least one of said anodes and an associated cathodehas a component generally parallel to said first surface; and at leastone second cathode electrode disposed on said second surface, saidsecond cathode electrode being maintained at a negative potentialrelative to said plurality of first surface anodes and cathodes, suchthat said second cathode electrode repels electrons generated near saidsecond surface, back towards said first surface.
 12. A radiationdetector according to claim 11 and wherein said negative potential isselected such that only electrons generated from incident radiation of apredetermined hardness are repelled back towards said first surface. 13.A radiation detector according to claim 11 wherein said substratecomprises cadmium zinc telluride.
 14. A method of performing photoncounting measurements on a radiation flux, comprising the steps of:providing a semiconductor substrate having a first surface for exposureto said radiation flux and a second surface opposite said first surface;disposing on said first surface a plurality of anodes and cathodes, suchthat an electric field produced between at least one of said anodes andan associated cathode has a component generally parallel to said firstsurface; and measuring the charge arising from an individual photonabsorption in said substrate.